The Mealy Oak Gall On Ornamental Live Oak In Texas

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The Mealy Oak Gall on Ornamental Live Oak in TexasPage 1 of 16MP-13157/78The Mealy Oak Gall on Ornamental LiveOak in TexasGordon W. Frankie, David L. Morgan, Mike J. Gaylor, James G. Benskin, Wayne E. Clark, Hal C. Reed and PhilipJ. /mp-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 2 of 16What are Galls?Galls are abnormal swellings of plant tissue induced by insects, bacteria, fungi, mites andnematodes. Insect-induced galls are the most common galls in urban areas. Among theinsects causing galls are certain moth caterpillars, beetles, flies, aphids and small wasps.The gall growth provides residing insects with food and shelter during certaindevelopmental stages. Galls have characteristic sizes, shapes and colors. In early days,when little was known about galls, naturalists named and described some galls accordingto plant growth patterns rather than by the insects causing them. This practice haschanged because some insect species induce more than one type of gall.Formation of Insect-induced GallsGall tissue is a plant product formed in response to a specific stimulus received from aninsect. The stimulus is thought to occur either when insect eggs are deposited in hostplant tissue or when eggs hatch and larvae feed on the plant (type of stimulus depends onthe insect species). In either case, chemicals, which are thought to secrete from specialinternal glands, interact with certain plant chemicals to produce abnormal plant growth.After a brief period of cell growth, gall development stops completely. The insect liveswithin the gall and feeds on gall tissue during its development. Once formed, thesegrowths discontinue utilizing host plant nutrients. Insect-induced galls are consideredharmless to the growth and development of almost all ornamental plants.The mealy-oak gall is one of the most common galls on live oak in Texas (figure 1). Thegall is induced by a small wasp, Disholcaspis cinerosa (figure 2). Gall-infested live oaktrees occur throughout Texas in natural and planted situations. The gall wasp also isreported from the same host in certain parts of Mexico and from western Louisiana. Thefollowing account summarizes 5 years of research on the biology, ecology and controlconsiderations of Disholcaspis cinerosa on ornamental live oak in several Texas /mp-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 3 of 16Disholcaspis cinerosa has 2 generations annually, each producing a different type of gallon live oak.Asexual generation. The asexual generation develops within mealy-oak galls on branchesand branchlets of host trees (figure 1). These spherically shaped galls are familiar topeople who live in cities where live oaks commonly are planted as ornamentals. Gallsrange from 1/8 to 1 inch in diameter and are first noticed in late summer or early fall,depending on locality. When first formed, they are small, light pink to pinkish brown andthe internal tissue is soft (figures 1 and 3).Disholcaspis larvae are believed to secrete chemicals that stimulate the plant to producethe spherical swellings. From late summer to late fall (figure 4) developing Disholcaspislarvae eat plant tissues within galls. From August to mid-October, internal gall tissue isyellow-green and moist (figure 3), while during late October and November the tissueturns brown and begins to dry (figure 4). During November, pupation occurs and shortlythereafter transformation to the adult stage takes place. During December and the firstpart of January, adults emerge by chewing holes at gall bases (figure 1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 4 of 16Emerging Disholcaspis adults of this generation are all females (figure 2) which do notrequire mating to lay viable eggs; hence they are asexual. Interestingly, this uniquereproductive habit also is found in many common and beneficial parasitic wasps and inmost aphids.The newly emerged Disholcaspis female seeks out swollen leaf buds on live oakbranches in which to deposit one or possibly two eggs per bud. Females may live 2 to 6weeks during which time each individual may lay up to 20 viable eggs. The frequentlyharsh weather at this time of year appears to have little adverse effect on the ins/mp-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 5 of 16activities of most females. Females die after eggs are deposited. Eggs remain in adormant state during the remainder of winter (January and February).During the first winter, recently abandoned spherical galls begin to weather and in theprocess change from pink to grey and become dry and hard (figure 1). If left undisturbed,they may persist on host trees for several years. In essence, grey galls are inactivestructures that serve to remind one of a past event. They actually assume a nonlivingstatus about 1 month (November) before adults begin emerging from galls.Sexual generation. As mentioned, females of the asexual generation lay their eggs inswollen leaf buds. These eggs hatch in early spring as leaf buds begin to open. Resultinglarvae develop quickly in new leaf tissue, and in the process they induce the formation ofa second type of gall. This new growth is beige and resembles a kernel of wheat in sizeand shape (figure 5). Larvae, pupae and adults complete development in a few weekswithin these small, obscure galls. The sexual generation differs from the asexualgeneration not only in gall type but also in that the spring galls produce adults of bothsexes. Sexual generation adults are about 1/3 the size of the asexual generation adults(figure 2).Immediately after emergence, male and female wasps mate; males then die. Matedfemales may live up to a week during which time each individual deposits about 15 eggsin the branches of live oak. The eggs pass through a 3- to 5-month dormant period. Afterthis period, larvae hatch and begin development in certain outer branch tissues, whichleads to formation of pink spherical galls (figure 1). Thus the annual cycle ofDisholcaspis is completed.Relation of Disholcaspis Development to Seasonal Changes in Live OakThe seasonal activity of Disholcaspis may be followed partially by observing certainobvious seasonal changes in the live oak tree. Simultaneously occurring seasonal eventsbetween Disholcaspis and live oak are shown in figure 6.Early winter (late November to early December). By early winter, freezing weather hasoccurred in most Texas areas. Leaf buds are swollen on most live oak trees although theold leaves have not begun to drop. These general climatic circumstances and host plantconditions coincide with the emergence of adults of the asexual generation.Late winter to early spring (February to March). Depending on locality, most live oaksare partially leafless by March and leaf buds have begun to break open. At the bases 315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 6 of 16expanding new leaves, rapidly developing Disholcaspis larvae are found. When newleaves are about three quarters expanded, the kernel-like galls of the sexual Disholcaspisgeneration may be observed on certain leaf terminals. Oftentimes galled leaf terminalsmay be recognized since infested terminals produce extra leaves, which are clusteredaround the bases of galls. Once the leaves are expanded fully (late spring), most of thesexual generation Disholcaspis adults emerge and most of the egg-laying stops.Summer to early fall. After new leaves are produced in spring (short shoots), anelongation of new branchlets takes place. Generally, the degree of shoot elongationdepends on the yearly rainfall pattern. Numerous long shoots usually result when rainfallhas been abundant during the year. At some time during shoot elongation, new asexualspherical galls begin forming on branches; however, no obvious tree activity patterncorresponds in time with the formation of these curious pink growths.In summary, most of the annual life cycle of Disholcaspis can be followed by merelyobserving changes in seasonal development of live oak trees.Variation in annual seasonal history of DisholcaspisWithin the same year. Seasonal history development of Disholcaspis in a given yeardiffers from locality to locality. The difference is most noticeable between cities locatedin the northern versus southern parts of the state, and it is related to the general climaticconditions and associated plant growth in the respective cities. Basically, southern Texascities experience more favorable growing conditions earlier in the year than the morenorthern cities. For example, populations of spring generation Disholcaspis begindevelopment earlier in the year in Houston than in Dallas. Also, new pink spherical gallsof the asexual generation always make their appearance earlier in Houston than in Dallas.However, adults of the same generation emerge from galls about the same time each yearin both cities.Year to year. In addition to difference among cities in a given year, the development ofDisholcaspis populations may vary by a month or more between years in a particular city.This difference is related t plant growth. When plant growth begins “early” in a givenyear, this means that certain associated insects will also usually be “early.” For example,in 1974 in Dallas, live oaks begin their seasonal changes “early” in the year. The “early”signs were noted in March when trees began flushing new spring leaves. At this time, thesexual generation of Disholcaspis also was beginning to develop. In 1975, Dallas live oaktrees did not begin flushing new leaves until April, which is considered “late” for thatcity. This “late” seasonal development of Dallas live oaks was accompanied by 15.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 7 of 16corresponding “late” seasonal development of Disholcaspis.Frequency of Gall-infested TreesAll live oak trees are not alike in their capacity to produce Disholcaspis galls. A total of170 live oak trees from city and shopping plazas in Dallas, Houston and San Antoniowere randomly selected to examine this possibly inherited capacity. The classes, light,moderate and heavy, were established arbitrarily to rank infestations of each tree.Infestation class was determined by sampling and counting all spherical galls (asexualgeneration) on 12 randomly selected branches (each ½ yard in length) from each tree.Survey results are summarized below. Numbers in parenthesis indicate range in numbersof galls recorded for each class:Number of trees recorded in each infestationInfestation classesclassLight (0 to 50)132Moderate (51 to28200)Heavy (201 to101,000)Total170The survey revealed that most trees (132 out of 170) were only lightly infested. In fat,118 out of the 132 trees in the light group had fewer than 25 galls per sample. Of theremaining two groups, usually only the heavy group causes concern to homeowners. Inthis group, only 10 out of 170 trees had high numbers of galls. The survey also indicatedthat trees classified as heavy usually are younger and smaller (less than 10 inches in trunkdiameter at breast s/mp-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 8 of 16Cycle of Abundance on Susceptible TreesExperimental studies indicate that the gall-forming capacity of relatively nonsusceptibletrees (light class) to Disholcaspis may be quite constant year after year. That is, if a treeshows little or no inclination to form galls after 2 to 4 years, it will probably p-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 9 of 16produce galls. In contrast, trees that are potentially capable of producing galls (mostlytrees in the heavy class) may experience a change in infestation class over several years.In this case, a potentially susceptible tree may pass through cycles of great gallabundance interspersed with periods of few or no galls.Long-term studies on highly susceptible trees (heavy class) suggest that a cycle of greatgall abundance may last 2 to 3 years. Periods of decline in gall numbers then ensue for 1to 3 years. At the end of the decline period, trees may not produce any Disholcaspis gallsin a given year. Length of the decline period is unknown; however, it is assumed that anappropriate period must pass before the gall-forming capacity of susceptible trees returns.Because of the cyclic nature of gall production, these trees are considered potentiallysusceptible.It is relatively easy for inexperienced persons to determine the state of infestation on anaffected tree. In late fall-early winter, select about eight branches from the outside of oakcrown (each branch should be about ½ yard in length and should be selected from adifferent side of tree). Remove all attached galls and divide into two groups: pink gals(new) and grey galls (old). Estimate the specific stage of the cycle by comparing thenumber of pink to grey galls:zzzIf the sample consists of mostly new pink galls, the infestation cycle is justbeginning.If most galls are grey, the tree is in a declining period of gall productionIf there are about equal numbers of pink and grey galls, the infestation is either justbeginning to increase or is in the early phases of decline. If most of the new pinkgalls in this group are large (3/4 inch to 1 inch diameter), the infestation is justbeginning on the tree (see below for further information on this last pattern).Two combined factors may be responsible for the decline in Disholcaspis galls onpotentially susceptible trees.zApparent live oak immunity to Disholcaspis. Experimental studies suggest that apotentially susceptible tree (as evidenced by the presence of old or new sphericalgalls) may be highly susceptible to gall formation for 2 to 3 years, after which thetree seems to lose most of its gall-forming capacity for a 2- to 3-year period. It isbelieved that a change in the tree’s chemistry may be responsible partly fordifferences in annual susceptibility to gall formation. Possible chemical changes inlive oak have not been investigated; however, if they do occur, they may be relatedto cycles of acorn production (this cycle varies from year to year) or possibly to 15.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexaszPage 10 of 16build up of defensive plant chemicals from a previous gall infestation. Other plantspecies react against insect infestations by producing defensive chemicals in highconcentrations. Only long-term studies, such as those presently underway at TexasA&M University, will reveal the causes of this change in live oak susceptibility.Parasites. Parasitic insects are believed to significantly reduce Disholcaspispopulations. Most parasitization takes place on individuals of the asexualgenerations spherical galls. The effect of the parasites is observed in the nextgeneration when fewer Disholcaspis are left to reproduce. Approximately 12parasitic wasp species have been reared from spherical Disholcaspis galls (figure 7).This is an impressive total since virtually all of the same parasites have been rearedfrom Disholcaspis galls in native stands of Texas live oak. The degree ofparasitization (percent parasitized Disholcaspis varies considerably from one tree tothe next. However, if the parasites occur in a particular area, they may parasitize upto 90 percent of the gall wasps on a given tree.Parasitized spherical galls can be recognized easily by their smaller size. Parasitizedgalls generally measure 1/8 to ½ inch in diameter, while unparasitized galls are ½ to1 inch in diameter. Disholcaspis usually are parasitized during the larval stage, atwhich time galls are small. A parasitized Disholcaspis larva stops producingchemicals that stimulate the tree to form gall tissue, thus stopping gall growth.Galls are More Abundant on Isolated TreesIsolated live oaks in urban environments often have greater infestations 315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 11 of 16Disholcaspis than live oaks occurring in groups. For example, young, potentiallysusceptible oaks planted in shopping plaza parking lots often are very heavilyinfested with galls. Reasons for this pattern are not fully understood. However, whenoak trees are first planted, the following ecological imbalance occurs. Beforeintroducing potentially susceptible live oaks into urban landscapes from nurseries,the trees have been carefully examined in accordance with state inspection standardsand treated for any symptoms of insect infestation or disease. When these “clean”oaks are planted in urban environments, they immediately become exposed to a widevariety of insects. In the case of Disholcaspis on an isolated oak, the gall inducersarrive before their natural enemies (wasp parasites). A year or more may be requiredbefore enough parasites find the galls and begin controlling Disholcaspis. Thus, acycle of initially high gall numbers followed by subsequent reduction in gallnumbers can be expected when susceptible host plants are moved from relativelyinsect-free situations to isolated urban landscapes. In essence, when man isolates liveoaks he is creating an island which is ecology imbalanced from the standpoint ofassociated insects.The Spherical Gall Insect ns/mp-1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 12 of 16In addition to the gall inducer and its parasites, the spherical gall has otherinteresting insects associated with it. Collectively, all of these insects are said toform a community. Like all communities (plant or animal), the community of insectsassociated with Disholcaspis has its own unique organization. The organization canbe described by examining the various stages of gall development and the insectsassociated with each stage.{ Stage 1: Newly developing spherical gal (August to early October). Severalspecies of small wasps are known to parasitize Disholcaspis larvae when thepink spherical galls first appear around August. Shortly after this period, agroup of plant-feeding wasps or “guests” begin to lay eggs in developing mantletissues of the gall. The larvae hatching from these eggs feed and develop inouter gall tissue without harming Disholcaspis (figure 8). From Septemberthrough December, many of these plant-feeders transform into adults andemerge from the mantle. It is not known where they go after leaving the gall.During stage 1, another group of insects is associated with the galls. Developgall tissue secretes sweet exudations which accumulate on the outer gall surface.These secretions are rich in sugars and serve as a food source for a wide varietyof insects including small flies, large predatory wasps, butterflies andhoneybees, with the latter usually the most abundant (figure 9). All of these -1315.html2/3/2004

The Mealy Oak Gall on Ornamental Live Oak in TexasPage 13 of 16visitors feed on the secretions until galls begin to dry in late fall.{ Stage 2: Mature spherical gall (late October to early January). A second groupof wasp species begins parasitizing mature larvae or pupae of Disholcaspisbefore adult emergence of gall inducer in December to early January (see figure7 for example). Disholcaspis adults emerge from nonparasitized galls inDecember to early January. After this event, the vacated gall is almostimmediately taken over by one of the following groups of beneficial insects:la

The mealy-oak gall is one of the most common galls on live oak in Texas (figure 1). The gall is induced by a small wasp, Disholcaspis cinerosa (figure 2). Gall-infested live oak trees occur throughout Texas in natural and planted situations. The gall wasp also is reported from the same host in certain parts of Mexico and from western Louisiana. The

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